The general aims of this project are to identify the mechanisms of light activated translation of organelle mRNAs. Previous biochemical and genetic experiments have identified key components required for translation of the chloroplast psbA mRNA. Under this proposal we will continue the characterization of psbA translation with the ultimate goal of identifying the complete set of proteins and RNA elements required for translation of chloroplast mRNAs. We will also investigate the molecular interactions of these components that result in light activated translation. The completion of this set of experiments should provide the basis for a detailed molecular model of translation of organelle mRNAs. Understanding chloroplast translation may also prove to be a paradigm for understanding prokaryotic translation, and for understanding the molecular basis of redox regulation of gene expression. These studies should also impact our ability to exploit plant chloroplasts as a vehicle for the expression of recombinant proteins, that potentially could be used as therapeutic agents in human health. The sets of experiments to address chloroplast translation can be divided into five specific aims: 1. Identify the mechanism by which the chloroplast poly(A) binding protein activates psbA translation. 2. Isolate a complete set of psbA mRNA binding proteins, and clone the encoding genes. 3. Identify the RNA elements required for psbA translation, and characterize the mechanism by which these elements function. 4. Identify the roles of psbA associated proteins in translation by genetic analysis. 5. Characterize the role of RNA elements and trans-acting factors in translation initiation complex formation